Heretofore, the following run flat tire has been widely known that enables a safety driving for a certain distance even when the tire is punctured. In this run flat tire, hard rubber layers each having a crescent shape in the lateral cross section are disposed respectively in sidewall portions of the tire (for example, see Patent Documents 1 and 2). When punctured to have a pressure of zero, the run flat tire of this type is allowed to run with the crescent hard rubber layers, which are disposed respectively in the sidewall portions, supporting the load of the vehicle applied to the tire.
However, the run flat tire of this type has the following problem because the load of the vehicle is supported by the hard rubbers on both of the left and right sides as described above. Specifically, in run-flat running where the tire is punctured, the area of the contact patch of the tire is reduced because a center portion, in the width direction, of a tread portion 4 is buckled as shown in FIG. 5. Accordingly, the maneuverability in running on a low friction road surface is deteriorated. In particular, in the case of a tire, such as a studless tire, that is designed to have a tread with low stiffness, this tendency of the deterioration is significant, so that the tire is eventually slipped on icy road surfaces.
As a countermeasure against this problem, the following approach has been proposed. In this approach, a belt addition layer formed of cords each extending at a large angle to the circumferential direction of the tire is disposed on the outer circumferential side of two belt layers disposed in a tread portion. The belt addition layer increases the compression stiffness, in the width direction, of the tread portion, so that the buckling phenomenon is suppressed (see, for example, Patent Document 3).
However, this approach causes a significantly excessive increase in stiffness of the tread because three belt layers are formed in total. As a result, this approach brings about a problem that the ride comfort in running on general road surfaces and the driving stability on snowy road surfaces at the time when the tire is not punctured are deteriorated.
Patent Document 1: Japanese Patent Application Kokai Publication No. 2003-94912
Patent Document 2: Japanese Patent Application Kokai Publication No. 2003-326924
Patent Document 3: International Publication No. WO2003/024727